If the Fed raises interest rates rates tomorrow (assuming they will) then theoretically the underlying value of every company should slightly decrease due to lower future cash flows and this should reflect falling stock prices. The prices of stocks are so heavily influenced by speculation and expectations so the ever so slight increase will cause much more dramatic effect than any real effect the interest rate hike has. I think Walstreet has probably already priced the stocks to include the rate hike for quite some time but I think your average stock market participant hasn't. This is why I think it'll result in a sharp downswing then upswing.